Optical fiber amplifiers are typically optically pumped. In a gain fiber, the core of the fiber is typically doped with one or more active species (e.g., rare-earth ion dopants). Typically, when one species is used (e.g., ytterbium), those ions absorb light of the pump wavelength and use the energy to amplify light of a signal wavelength by stimulated emission. Typically, when two species are used (e.g., erbium and ytterbium), the ions of one species absorb light of the pump wavelength and transfer that energy to the second species, and the second species uses the transferred energy to amplify light of a signal wavelength by stimulated emission.
U.S. Pat. No. 5,088,095 to Martin Zirngibl titled “GAIN STABILIZED FIBER AMPLIFIER” is incorporated herein by reference. This patent describes an optical fiber doped with a rare earth element and coupled to be pumped with a laser is coupled to an optical feedback loop. The feedback loop couples the output signal of the fiber amplifier to the input of the fiber amplifier. A narrow bandwidth filter coupled to the feedback loop allows a selected wavelength of the amplified spontaneous emission to pass from the output of the fiber amplifier to the input of the fiber amplifier. The feedback signal has a wavelength which is different from that of the pump signal and the wavelengths of the signals to be amplified. In operation, when bursts of optical signals from at least two discrete word or frequency division multiplexed channels are amplified in the fiber amplifier, the undesired fluctuations of gain of the output signals normally due to transient saturation of the erbium-doped filter amplifier are substantially eliminated.
U.S. Pat. No. 5,982,790 to William Mark Grossman et al. titled “SYSTEM FOR REDUCING PULSE-TO-PULSE ENERGY VARIATION IN A PULSED LASER” is incorporated herein by reference. This patent describes a system and method for reducing pulse-to-pulse energy and peak power variation in various types of pulsed lasers, and in Q-switched lasers in particular. The system of invention has a laser cavity with a lasing medium pumped by a pumping device for delivering to the medium a pumping energy Epump. The system further includes a detection device and circuitry for determining the pulse magnitudes Mi of laser pulses i, such as peak pulse amplitudes Ai, pulse energies pulse widths Wi or other pulse metrics. According to the method of invention, a feedback mechanism which is in communication with the pumping device ensures pulse-to-pulse stability by increasing the pumping energy Epump when pulse magnitude Mi of laser pulse i exceeds a mean pulse magnitude [M] and decreasing the pumping energy Epump when Mi is less than [M]. Alternatively, the feedback mechanism is in communication with the switching device which controls that variable loss factor of the Q-switch. Pulse-to-pulse peak and energy stability is achieved by decreasing the variable loss factor when Mi of pulse i exceeds the mean [M] and increasing the variable loss factor when Mi of laser pulse i is less than [M].
U.S. Pat. No. 6,064,514 to Yasuhiro Aoki et al. titled “OPTICAL SURGE PREVENTING METHOD AND SYSTEM FOR USE WITH OR IN A RARE EARTH DOPED FIBER CIRCUIT” is incorporated herein by reference. This patent describes optical surge preventing systems and methods for rare earth-doped optical fiber amplifiers. The systems are so arranged as to cause any signal existing in a doped fiber section so as not to over excite the rare earth elements in the doped fiber section. In one embodiment, a background signal light with such a wavelength as to cause induced emission in the doped fiber section is always coupled into the doped fiber section regardless of whether a message signal light exists or not. The background signal light may be generated by any suitable light source or a light spontaneously emitted from either end of the doped fiber section. In another embodiment, a dummy signal light is coupled into the doped fiber section if the level of the message signal light becomes less than a predetermined value. Coupling of the background signal light or the dummy signal light into the doped fiber section may be done from either of the message signal input and output sides.
U.S. Pat. No. 7,027,199 to Jay Johnson titled “AOM MODULATION TECHNIQUES FOR FACILITATING PULSE-TO-PULSE ENERGY STABILITY IN LASER SYSTEMS” is incorporated herein by reference. This patent describes digital control of frequency and/or amplitude modulation techniques of an intracavity and/or extracavity AOM (60) facilitate substantially full extinction of a laser beam (90) to prevent unwanted laser energy from impinging a workpiece (80); facilitate laser pulse amplitude stability through closed-loop control of pulse-to-pulse laser energy; facilitate beam-positioning control including, but not limited to, closed-loop control for applications such as alignment error correction, beam walk rectification, or tertiary positioning; and facilitate employment of more than one transducer on an AOM (60) to perform any of the above-listed applications.
U.S. Pat. No. 7,254,147 to Katsuichi Ukita titled “LASER CONTROL METHOD LASER APPARATUS LASER TREATMENT METHOD USED FOR THE SAME LASER TREATMENT APPARATUS” is incorporated herein by reference. This patent describes a laser controlling method that can generate laser of stable laser pulses, and eliminate useless time from a machining procedure. The method uses a gain medium and a Q-switch, and emits exciting light to the gain medium, thereby setting the Q-switch in a continuous oscillation mode, and prepares a given Q-switch pause time before a laser pulse is generated. When the continuous oscillation is kept going longer than a given time, the control method sets a Q-switch pause time for obtaining a first laser pulse to be different from a Q-switch pause time for obtaining a second laser pulse and onward.
U.S. Pat. No. 7,313,155 to Liyue Mu et al. titled “HIGH POWER Q-SWITCHED LASER FOR SOFT TISSUE ABLATION” is incorporated herein by reference. This patent describes a high power Q-switched, intracavity frequency-doubled laser for laser ablation of soft tissue. Operating a high power Q-switched laser in a frequent on-off mode is highly desirable for laser prostatectomy. Giant first pulse may occur when a Q-switched laser is switched from laser-ready mode to pulse-on mode due to sudden depletion of stored energy in the gain medium. Such a giant first pulse may cause power damage of intracavity optics. Besides, temperature shock induced by sudden onset of a high power pulse train may cause optical damage on surface coating of intracavity optics. The present invention contemplates to suppress these giant first pulses and temperature shocks through pre-lasing and ramping profile of laser parameters. Reliable and frequent on-off operation of a diode-pumped, Q-switched, frequency-doubled Nd:YAG laser is demonstrated for output power up to 100 W.
U.S. patent application Ser. No. 11/484,358 filed Jul. 10, 2006 by Angus J. Henderson and titled “APPARATUS AND METHOD FOR PUMPING AND OPERATING OPTICAL PARAMETRIC OSCILLATORS USING DFB FIBER LASERS” (now U.S. Pat. No. 7,620,077) is incorporated herein by reference. This application describes an optical parametric oscillator (OPO) that efficiently converts a near-infrared laser beam to tunable mid-infrared wavelength output. In some embodiments, the OPO includes an optical resonator containing a nonlinear crystal, such as periodically-poled lithium niobate. The OPO is pumped by a continuous-wave fiber-laser source having a low-power oscillator and a high-power amplifier, or using just a power oscillator. The fiber oscillator produces a single-frequency output defined by a distributed-feedback (DFB) structure of the fiber. The DFB-fiber-laser output is amplified to a pump level consistent with exceeding an oscillation threshold in the OPO in which only one of two generated waves (“signal” and “idler”) is resonant within the optical cavity. This pump source provides the capability to tune the DFB fiber laser by straining the fiber (using an attached piezoelectric element or by other means) that allows the OPO to be continuously tuned over substantial ranges, enabling rapid, wide continuous tuning of the OPO output frequency or frequencies.
As used herein, the optical signal (also called the signal, the seed signal, or the seed) is light of the signal wavelength being amplified or of the laser output (and may or may not be modulated with information), and the optical pump (also called the pump) is light of the pump wavelength used to input optical energy and power to the optical amplifier or laser by exciting an active species or dopant. As used herein, absorbing/absorbent material and/or dopants each mean a species (such as rare-earth ions) that are added to at least a portion of an optical fiber to absorb at least one wavelength without substantial re-radiation of stimulated emission. As used herein, active or signal dopants each mean one or more species (such as rare-earth ions) that are added to at least a portion of an optical fiber to absorb at least a pump wavelength and to provide stimulated-emission amplification of a signal wavelength (i.e., a species that absorbs pump light and amplifies signal light).
When amplifying fibers are used to amplify pulsed signal light, the pump light is sometimes fed into the fiber for a period of time before the signal pulse (e.g., in some embodiments, the pump laser is left on continuously), and the optical pump energy at a pump wavelength is absorbed by the active species such that between pulses, it builds up over time in the fiber. This stored energy is released by amplifying a seed pulse of the signal wavelength. This can lead to short signal pulses of several kilowatts peak power even if the continuous-wave (CW) pump power is less than 10 watts.
However, such systems are susceptible to having different amounts of gain and power in their output pulses. Particularly for systems that utilize nonlinear frequency conversion to generate alternate wavelengths, the pulse stability can deteriorate significantly due to the nonlinearities of the frequency-conversion process. What are needed are improved methods and apparatus for stabilizing the gain and the per-pulse power in pulsed-laser systems.